The demand for wooden raw materials is constantly on the rise. Meeting this interest requires developing new timber supply chains, tackling illegal trade, using timber more efficiently, and enhancing the service life of wood-based products. From this point of view, sweet chestnut wood can be successfully used, alone or mixed with poplar, to manufacture Oriented Strand Board (OSB). This article assesses the fungal resistance of OSB/3 (load-bearing boards for use in humid conditions) made with different fractions of poplar (P) and sweet chestnut (SC) wood, in order to take advantage of the protective action of the tannins contained in the latter. Four types of OSB/3 were manufactured on an industrial scale: 100% P; 80% P and 20% SC; 50% P and 50% SC; 100% SC. The durability of the experimental boards was determined in accordance with the EN 113-3 standard (2023). No significant differences in mass loss were found for OSB/3 with different sweet chestnut wood fractions. Instead, the mass loss of these panels was significantly lower (i.e. fungal resistance was higher) than that of the poplar OSB/3 and poplar plywood tested for comparison. Based on the results, the use of sweet chestnut wood has interesting potential for the manufacturing of OSB/3 made with adequate gluing quality, suitable for applications in environments where fungal decay may occur.